Hydroprocessing reactions are slow, and it is therefore important to have catalyst with a large internal surface area. In order to maximise the area available the porosity of the catalysts should be as large as possible. For catalysts that are used to hydrotreat heavy oils, the porosity becomes even more important, as the pore space is used for accumulating metals and coke from the feed. For these catalysts, the maximum porosity that can be obtained determines not only the activity of the catalyst but also the life as the storage capacity for coke and metals are related to the porosity. The strength of the catalyst shape (SCS) decreases as the porosity is increased. For many applications SCS of the catalyst particles is a limiting factor of the catalyst porosity.
Heteroatoms can be removed from mineral oils by hydrotreating the oil over a hydroprocessing catalyst at elevated temperature often in the range of 250-450xc2x0 C., with hydrogen at a partial pressure of 15-200 bar and with a LHSV in the range of 0.1-5.0 hxe2x88x921. The exact conditions will depend on the required degree of conversion, on the type of feedstock that are to be treated and on the desired run length. For a refiner, the run length is a very important consideration. A shorter run length means high costs due to a higher rate of catalyst replacement, and relatively more downtime (i.e. time off-stream) for catalyst change-out with a resultant loss of revenue due to reduced production. The catalyst used in the hydrotreating reactor may be any catalyst used for hydrotreating of a hydrocarbon feed being known in the art. The catalyst contains at least one metal on a porous refractory inorganic oxide support. Examples of metals having hydrotreating activity include metals from groups VI-B and VIII e.g. Co, Mo, Ni, W, Fe with mixtures of Coxe2x80x94Mo, Nixe2x80x94Mo and Nixe2x80x94W preferred. The metals are employed as oxides or sulphides. Examples of porous material suitable as support include alumina, silica-alumina, alumina-titania, natural and synthetic molecular sieves and mixtures hereof, with the alumina and silica-alumina being preferred. The catalyst is thereby usually in shape of cylindrical, trilobal, quardrolobal or spherical geometries. Size and shape of the catalysts particles depends on the actual application. In many processes, applications of catalysts are limited by diffusion, in these cases the particle size has to be as small as possible. In upgrading of heavy oils containing trace amounts of metals it is in particular important to use catalysts in form of small particles as diffusion of the metal bearing species are under strong diffusion control. In course of the process the metals are accumulated in the pore system of the catalyst and the effective life of the catalyst is determined by its ability to continue to remove metals. The capacity for metal removal depends on distribution of the metals in the pore system and on the space available in the catalyst for storing coke and metals. The highest metal storage capacity is obtained when the metal deposited from the feed is distributed evenly over the cross section of the catalyst pellets. An even distribution can be obtained by using catalysts with large pores, a bimodal pore system with large macropores or by using catalysts in the form of small extrudates. A high volume for storing coke and metals can be made by increasing the porosity of the carriers for the catalysts, using conventional manufacturing techniques for catalyst carriers.
The steps that are taken to increase the porosity of catalysts reduce the strength of the individual particles. The smaller the particles the less the strength, high temperature treatment for increasing the pore volume of the catalyst reduces strength of the particles and the higher the porosity the lower the strength. Impact of porosity on strength is well known and described by a relationship developed by Rytzkevitz (J. Amer. Ceram. Soc., 36 (2), pp 65-68 (1953)):
SCS=exp(xe2x88x92bxc3x97"THgr"); 
wherein
"THgr" is the porosity of the catalyst in cc pore volume/cc particle volume, and
b is constant for porous materials between 5 and 7.
In the above relationship constant b depends on both properties and geometry of the porous catalyst body. For a given material the strength changes when changing the shape. Shapes that result in increased strength allow use of materials with larger porosity at a desired mechanical strength.
The general object of the invention is to provide catalysts with shapes having improved mechanical strength determined by the use of the ASTM D4179 Standard Test Method for Testing Pellet Strength of Formed Catalyst Shapes (SCS) and high porosity. Those catalysts are useful in heavy oil up-grading where the effective life of the catalysts are determined by ability of the catalyst to store contaminants such as metal and coke. In ebullating bed service the use of the geometry according to the invention improves mechanical strength of the catalyst without changing the ebullating characteristics of the catalysts as the geometry is close to the usual cylindrical extrudates that are used for these types of processes.
Accordingly, this invention provides a hydrotreating catalyst containing one or more metal oxides and/or sulphides selected from Group VI-B and Group VIII of the Periodic Table supported on a porous refractory carrier being in form of an angular extrudate with rounded edges.